1. Field of the Invention
This invention relates to the simultaneous manufacture of wet process phosphoric acid and sodium silico-fluoride. More particularly, this invention relates to the recycling of waste-water streams, such as gypsum pond water, to recover P.sub.2 O.sub.5, fluoride and silica values contained therein and reduce pollution of the waterways and the air.
2. Description of the Prior Art
The manufacture of wet process phosphoric acid employs voluminous amounts of water. For example, large amounts of water are normally required for sulfuric acid dilution and product recovery stages. During such acid production, a large portion of process water ends up in an aqueous slurry containing considerable amounts of gypsum, sulfuric acid, P.sub.2 O.sub.5, as well as varying minor amounts of fluorine, silicon and other impurities. This aqueous slurry is routinely passed to one or more large gypsum ponds where insoluble materials settle out and a waste-water solution (i.e., also known as gypsum pond water) is formed containing soluble P.sub.2 O.sub.5, fluorine and silicon values along with other impurities.
Because of the relatively large amounts of such gypsum pond water produced at each facility, it is usually necessary to re-use at least a portion of the gypsum pond water to recover the economically valuable P.sub.2 O.sub.5, to reduce fresh water requirements, and to avoid polluting other water sources in the area.
In particular, it is known to use gypsum pond water in the dilution of sulfuric acid, such as is described in U.S. Pat. Nos. 3,498,746 and 3,498,747, issued to R. G. Smaltz et al on March 3, 1970 and in U.S. application Ser. No. 697,179, filed by S. Kongpricha on June 17, 1976 now U.S. Pat. No. 4,117,091. All three of these references are incorporated herein by reference in their entirety. However, while these three prior art teachings describe methods for recovering P.sub.2 O.sub.5 values from gypsum pond water, they do not provide any means for recapturing fluorine and silicon values contained in the gypsum pond water without the employment of more water. Furthermore, the dilution of the concentrated H.sub.2 SO.sub.4 by the processes described in these prior art references may cause SiF.sub.4, and possibly HF, gases to be evolved, thereby possibly causing air pollution. Still further, it has been found that scrubbing the evolved SiF.sub.4 and HF gases with water, either fresh water, process water or waste-water streams such as gypsum pond water, will not work well since the water cannot be put back into the phosphoric acid process without diluting the acid product. Also, such scrubbing water cannot be readily disposed of bacause of the fluorine and silicon values contained therein. Finally, return of such scrubbing water back to gypsum ponds would be the remaining choice, except that the residual fluorine and silicon values would be lost.
Therefore, it would be advantageous if an alternative scrubbing liquid besides water could be employed in the scrubbing of SiF.sub.4 and HF gases evolved during the dilution of the concentrated H.sub.2 SO.sub.4 with gypsum pond water whereby acid dilution and disposal problems may be eliminated and additional fluorine and silicon values may be recovered. The present invention provides such an alternative.